New string prototype method and object lock fix.

* Added `format()` method to string prototype. Closes #13
* Locked objects may not be modified in any way so as to be consistent with the behaviour of the `clear()` and `add()` prototype methods.
Fix code style hiccup.
Rearrange logic in `StringPrototype::Format`.
Remove argument count check.
Merge pull request #14 from Syke94/master

New string prototype method and object lock fix.